The present relates to a cell for the electrolytic purification of aluminum, comprising a trough having an outer steel tank, a refractory lining and a carbon base containing the anodically connected iron bars; a melt of an aluminum alloy doped with a heavy metal or heavy metals, which has a density .rho..sub.1 and forms the anode; a layer of molten electrolyte material resting on the anode and having a density .rho..sub.2 ; a top layer of molten extra-high purity aluminum, which has a density .rho..sub.3 and forms the cathode; and graphite cathodes which are fixed to the cathode cell structure and dip from above into the extra-high purity aluminum; .rho..sub.1 being greater than .rho..sub.2 which is greater than .SIGMA..sub.3.
The electrolytic refining of aluminum, like all electrolytic refining processes, is based on the fact that the, relative to aluminum, comparatively
base components (for example, sodium, lithium and calcium) of the alloy employed, while dissolving anodically in the aluminum, cannot be deposited at the cathode, and PA1 the noble components (for example, copper, silicon, iron and titanium) do not dissolve anodically and thus stay behind in the anode metal, with formation of liquation crystals. PA1 the heavy bottom layer which consists customarily of an Al/Cu/Si/Fe alloy and whose surface is at the same time the anode; PA1 the electrolyte layer consisting of the fluorides and/or chlorides of alkali metals and alkaline earth metals; and PA1 the refined aluminum, the third (top) layer whose lower surface forms the cathode. PA1 electrolytes having a higher electric conductivity are employed and/or PA1 the interpolar distance, that is the thickness of the electrolyte layer, is lowered.
The three-layer refining cells for aluminum, which have been known since the beginning of this century, contain three liquid layers:
When the electrolysis direct current is applied, the aluminum is oxidized at the anode to trivalent aluminum ions; these ions migrate to the cathode where they are reduced back to aluminum.
Through the forehearth of the cell, which is at a lower temperature than the 750.degree. C. that is customary for the refining of aluminum, the impurities that have crystallized out, particularly intermetallic products of Al, Cu, Fe and Si, known as liquation crystals, are removed.
The energy consumption of the three-layer refining cell for aluminum is relatively high. Typical values for the cell voltage are about 5.5 V, for a current efficiency of about 95 to 97%. This gives an energy consumption of approximately 17 to 18 kWh/kg of refined aluminum. From a purely physical point of view, the energy consumption of the aluminum-refining electrolysis can be reduced essentially by two measures:
The electrolyte layer, which customarily has a thickness of 10 to 20 cm, cannot, however, be reduced indefinitely without the risk of mechanical contamination of the refined aluminum layer through contact with the anodically connected aluminum alloy.
U.S. Patent Specification Nos. 4,115,215 (Re 30,330) and 4,214,956 propose an apparatus for the electrolytic refining of aluminum which deviates from the three-layer method that has been customary so far. The aluminum alloy to be purified is placed in a vessel-shaped diaphragm which is surrounded by a molten electrolyte. The density .rho..sub.2 of this electrolyte, in contrast to the three-layer refining cell, lies below that (.rho..sub.3) of the extra-high purity aluminum. By using a diaphragm that is impermeable to the aluminum alloy to be refined, the problem of mechanical contamination can be solved. The diaphragm material used is "Poros Carbon PC-25" from UNION CARBIDE Corporation, having a porosity of 48% and a mean pore diameter of 0.12 mm.
The requirements for the diaphragm according to the two U.S. Patent Specifications may be characterized as follows: on the one hand, the diaphragm of an aluminum refining cell has to be impermeable to the aluminum alloy employed and, on the other hand, it is to have the lowest possible electric resistance. Obviously, these two requirements are mutually opposed with respect to the thickness and porosity of the diaphragm. Thus the properties of the diaphragm are of critical importance for the specific energy consumption of the refining cell.
Not only do the higher-melting Al/Si/Fe compounds formed during the electrolytic refining of aluminum alloys reduce the efficiency, that is to say the ratio of the aluminum recovered to that employed, but the liquation of such alloys can lead to the clogging of the finely porous diaphragm. At any rate, by using such a refining cell with diaphragm, the specific energy consumption can be taken to values somewhat below those attained in the electrolytic production of aluminum by means of modern Hall/Heroult cells.
The inventors have set themselves the object of providing a cell for the electrolytic purification of aluminum having a low diffusion resistance and low electric resistance, by means of which cell high metallurgical efficiency is achieved. A three-layer refining cell is to be employed which, due to the low electric resistance intended, is provided with better thermal insulation.